Transcutaneous oxygen partial pressure (tcpO.sub.2) measurements have hitherto usually been carried out by polarography/amperometry, for example using a two-electrode arrangement. However, a particular problem which arises with this is that a silver/silver chloride layer is formed by anodic chlorination, because the anode and the reference are combined as one electrode composed of silver when a chloride ion-containing electrolyte is used. An electrode arrangement of this known type is also unsatisfactory with regard to its long-term operating stability. Electrolytic silver transport to the cathode takes place, with deposition of silver on the cathode, especially with so-called microcathodes, resulting in a change in its area and thus in the pO.sub.2 measuring current. Occasionally, there is formation of rapidly growing silver dendrites, especially when electrolyte layer thickness is only a few microns.
On simultaneous transcutaneous measurement of the carbon dioxide partial pressure (tcpCO.sub.2) by the Stow-Gertz-Severinghaus method in the same electrolyte using a combined measuring sensor there is, moreover, an effect on the pH by the cathodic production of hydroxyl ions.
A known three-electrode arrangement for polarographic/ amperometric pO.sub.2 measurement is available for eliminating these effects, as has been described, for example, by Severinghaus for a combined measuring sensor in "Journal of Applied Physiology", Vol. 51/1981, pages 1027-1032 (cf. in particular FIG. 2 on page 1028). In a multiple electrode arrangement of this type the function of anode and reference electrode are divided, so that there is, in particular, some freedom in the choice of material for the anode so that, for example, platinum, or gold can be used or, inter alia, the same material as that for the cathode. It is possible in this way to bring about consumption of the same number of hydroxyl ions at the anode as have been generated at the cathode.
However, with the thin electrolyte layer which is necessary for transcutaneous measuring sensors it is found that, in contrast to the customary dimensions, the reference electrode must, for reasons of stability, especially in the said combined sensor, be designed with a relatively large surface area, whereas the counter electrode (anode) ought rather to have a smaller area. However, once again a certain silver ion concentration develops in the electrolyte, which again results in silver deposits on the cathode. The measuring faces of sensors of this type must therefore be polished from time to time, and especially before each new series of measurements. This also applies to the abovementioned combined measuring sensor described by Severinghaus.